Dispersion Preparation

B. Optimization of ZnO NW dispersions for SNW device fabrication

Fig. B.2: FESEM image showing separation tendency of ZnO NWs in four different concentrations of disper- sions of ZnO NWs in DI water added with SDS

targeted to be placed in the channel (gap) between the two electrodes such that it can properly bridge the gap between the two electrodes and connect them properly. To separate ZnO NWs, one of the preferred methods will be to make a solution or dispersion of ZnO NWP and drop cast the same over desired location. Generally, ZnO NWP is not soluble in many solvents except some acids. Moreover, acids evaporate fast and also need more precaution during experimentation. Therefore, experimenting with ZnO NW dispersions drops is preferred here over ZnO NW solutions.

1 mg of ZnO NWP was taken and dispersed in four different concentrations (5 mL, 10 mL, 15 TH-2495_146102016

B.2 Dispersion Preparation

Table B.1: Count of distinguishable and separated individual ZnO NWs entities in each dispersion drops Drop columns/

ZnO dispersions C1 C2 C3

D5 (5 mL) 9 2 10

D10 (10 mL) 3 4 7

D15 (15 mL) 2 3 6

D20 (20 mL) 1 2 2

mL and 20 mL) of low surface tension de-ionized (LST-DI) water. For convenience, we have named four dispersions as D5, D10, D15 and D20 respectively. A low surface tension DI water (LST-DI) was prepared by dissolving 1 mg of sodium dodecyle sulfate (SDS) in each DI water sample. Each of four dispersions were treated with vortex generator for 5 minutes for proper mixing of components and 0.5 µL of each dispersion was drop casted at three different locations over a cleaned silicon substrate as shown in Figure C.1. In this way, three different columns of dispersion drops were casted on silicon substrate as shown in Figure C.1 as C1, C2 and C3. This sample was annealed till 210^◦C with a ramp rate of 2^◦C/min starting from 60^◦C. The so annealed sample was imaged with field-emission scanning electron microscope (FESEM) from JEOL (Model: JSM-7610F) to observe the presence of ZnO NWs in each of dispersion drops. SDS helps to reduce the surface tension of the water [185–187] and hence resist the agglomeration rate of NWs resulting in a improved level of NWs separation as shown in Figure B.3.

All the FESEM images are taken near the centre of each drop. Images which are taken at least with 1000X for all columns were considered for comparison and analysis for each drop. It is observed that number of ZnO NWs over the imaged surface decreases as the concentration of the LST-DI water increases in accordance with the level of dilution. Hence, D5 contains maximum number of ZnO NWs while D20 contains least number of ZnO NWs. In D5, we also observe small clusters of NWs as sown in Figure B.2. Also, it is observed that NWs get agglomerated due to annealing to form random nanowire clusters. It is observed that two or more than two smaller ZnO NWs get stacked over each another to form a bigger single ZnO nanowire with length ∼ 5 to 10 µm. Moreover, D20 comes out as the best dispersion to be used for further experimentations for device fabrication. Table 1 shows the count of completely separated and easily distinguishable ZnO SNWs entities in each column of all four dispersions. It has been observed that C1 and C3 show a continuous decline in the count of a ZnO SNW as the amount of LST-DI water is increased from 5 mL to 20 mL which hints to the TH-2495_146102016

B. Optimization of ZnO NW dispersions for SNW device fabrication

Fig. B.3: Optical image showing a portion of ZnO NW dispersion drop in (a) DI water only (b) DI water + SDS after heating. ZnO NWs are in black colour

Fig. B.4: Optical image showing a portion of 2 µL drop-casted ZnO NW dispersion (a) without acetic acid (b) with acetic acid in 1:1 proportion

fact that ZnO NWs show higher separation tendency with increased dispersion medium concentration i.e. higher dilution ratio. The probable reason can be the increase in the repulsive force exerted by water molecules on the ZnO NWs. Moreover, C2 also shows a decreasing trend in the count of ZnO NW like C1 and C3 for D10, D15 and D20 except for D5 case where the scanned area needs to be changed for better results. Further, to achieve the target of distributing separated-single nanowires over the substrate, a rapid annealing, direct heat treatment or addition of other materials such as acids or base to the DI water based NW dispersion is expected to give improved results as it provides less time for water molecules to interact with ZnO NWs and hence reduces the chances of corresponding agglomeration.

TH-2495_146102016

B.2 Dispersion Preparation

B.2.1 Results and Discussion

B.2.2 Effect of acid addition to ZnO NW dispersion in LST-DI water

When ZnO NW powder is dispersed in LST-DI water, all the powder particles got agglomerated and assembled at one place which showed that water molecules exert repulsive force on ZnO NWP particles which created the assembly of larger and agglomerated ZnO NW clusters as shown in Figure C.2(a).

Therefore, to achieve higher degree of NW separation, mixing acids in ZnO NW dispersion in LST-DI can be a viable option [133, 134]. DI water molecules show repulsive response to ZnO NWs and acetic acid molecules are expected to attract the NWs. Therefore, adding acids in optimal concentration to LST-DI water is expected to result in a dispersion system where ZnO NWs will experience two different types of forces (one repulsive and other attractive) which will help them to remain separated.

For this experiment, LST-DI water is prepared simply by adding 1 mg of SDS to 5 mL of DI water. ZnO NW dispersion is by prepared by adding 2 mg of ZnO NW powder in 1200 ÂţL of LST-DI water. The reason to experiment with a very small concentration of ZnO NW dispersion in ‘µL‘ level as compared to previous ‘mL‘ level is to keep the density of ZnO NWs high so that the effect of acids can be observed clearly. 2µL of above prepared ZnO NW dispersion was drop-casted on to a cleaned silicon substrate and initial image is captured with camera of Material Printing System (MPS) [55].

The drop-casted ZnO NW dispersion is in white colour as seen from naked eyes. Then, 2µL of acetic acid, hydrochloric acid (HCl) and sulphuric acid (H₂SO₄) is added to the already drop-casted ZnO NW dispersion to keep the proportion same (1:1). It is observed that (H₂SO₄) dissolves ZnO NWs in all the proportions. HCl also partially dissolves ZnO NWs, however, these NWs turned into micro-particles when heated at ∼220^◦C as shown in Figure B.5.

When acetic acid is added in 1:1 proportion with NW dispersion, it is observed that within few seconds, the white colour of the dispersion changed to colourless and transparent like LST-DI water as shown in Figure C.2(b). The agglomerated and dense NW clusters got fragmented into smaller and highly separated ZnO NW clusters and the ZnO NW dispersion seemed almost transparent which confirms expected separation tendency between ZnO NWs as hypothesised by us earlier. The ZnO SNW with length < 10 µm are almost invisible at this scale as shown in Figure C.2(b). After drop- casting 0.5µL of above composition on a silicon substrate and heating immediately at 120^◦C resulted into the formation of long (>10µm), sharp-edged and highly separated nanowires and micro-wires are formed as shown in Figure B.6. Further, acetic acid was added in different volume proportions such as TH-2495_146102016

B. Optimization of ZnO NW dispersions for SNW device fabrication

Fig. B.5: Optical image showing (a) HCl partially dissolving ZnO NWs (b) NWs converted to micro-particles at high temperatures

1:1 (0.5µL acetic acid in 0.5µL of NW dispersion), 1:10 (0.5µL acetic acid in 5µL of NW dispersion) and 1:40 (0.5 µL acetic acid in 20 µL of NW dispersion) as shown in Figure B.7. It is observed that acetic acid addition to ZnO NW dispersion in different proportion is resulting long and sharp-edged NWs and therefore, has capability to modulate the morphology of already synthesized ZnO NWs due to its different pH range as compared to H₂SO₄ or HCl [133, 134, 188, 189]. The acetic acid dissolved all the ZnO NWs in those dispersions in which SDS was not added. This is a strong evidence that the it is the combination of acetic acid and SDS in proper ratios, that is mainly responsible for the creation of long, sharp-edged and highly distinguishable ZnO NWs [190, 191]. However, the dispersion containing acetic acid also converts nanowires and micro-wires into ZnO micro-particles when exposed to high temperatures near∼ 220^◦C as shown in Figure B.7(f).

B.2.3 Effect of base addition to ZnO NW dispersion in LST-DI water

To observe the effect of adding base (higher pH substance) on the morphology and separation tendency of ZnO NWs, 0.5 µL of ammonium hydroxide (NH4OH) was added in 0.5 µL (1:1), 10 µL (1:20) and 20 µL (1:40) of ZnO aqueous dispersion (1 mL DI water + 3 mg ZnO NW powder). As observed from the optical images taken from MPS microscope, NH₄OH shows no tendency to either dissolve the NWs or separate them. It forms a ring of ZnO NW mat-cluster of width near 25-50 µm with small, single and unmodified ZnO NWs scattered randomly along the periphery of the ring as shown in Figure B.8. The width of NW cluster ring increases from ∼ 32 µm to 60 µm as shown in Figure B.8(a,b).

TH-2495_146102016

B.2 Dispersion Preparation

Fig. B.6: Optical image showing sharp and long ZnO micro-wires formation after acetic acid addition in 1:1 (a) at periphery of drop (b) in middle portion of drop. AA: Acetic acid

Fig. B.7: Optical image showing sharp and long ZnO micro-wires formation after acetic acid addition in (a) 1:1 (a) 1: 10 (c) 1:40 proportion (d) 1:10 drop periphery (e) zoomed-in image of encircled region in (d) (f) ZnO nanowires converting to micro-particles at higher temperatures. AA: Acetic acid NWD: NW dispersion

B.2.4 Effect of annealing temperature and annealing rate

To observe the thermal stability of ZnO NWs in general, the earlier sample drops (without any acid or base) in LST-DI based dispersions are heated to first 180 ^◦C and then to 220^◦C. The ZnO NWs are thermally stable and their morphological properties are retained till 220^◦C as shown in Figure B.9.

However, it is also observed that SDS (as seen in blue color in Figure B.9(a) and Figure B.8) evaporates at temperatures near 220^◦C. Further, NW dispersion added with NH₄OH is also annealed to 180^◦C to TH-2495_146102016

B. Optimization of ZnO NW dispersions for SNW device fabrication

Fig. B.8: Optical image showing formation of ZnO NW mat-cluster ring after NH4OH addition to ZnO NW aqueous dispersion in (a) 1:20 (b) 1: 40 after heating the drop at 180^◦C

Fig. B.9: Optical image showing effect of annealing temperature on ZnO NWs at (a) 180^◦C (b) 220^◦C

220^◦C for 5 minutes and was found that there is no effect of NH₄OH on the morphology of ZnO NWs as observed in the case of acids which converted ZnO nanowires/microwires to ZnO micro-particles.

It is not only the optimized concentration of acetic acid and SDS addition in ZnO NW aqueous dispersion helps the formation of long, sharp-edged NWs but also the annealing rate. In first case, when acetic acid is added in proper proportion and annealing is started from 80^◦C, it is observed that acetic acid dissolves most of the ZnO NWs completely. However, when a direct annealing is done at 120^◦C to the NW dispersion in LST-DI, immediately after the addition of acetic acid drop, the formation of long, sharp-edged and highly separated NWs takes place.

Figure B.10 and Figure B.11 show the initial stage development of printed micro-resistors of ZnO SNW using MCP technology [36] and the I-V characteristics of a printed resistor and diode of ZnO NW respectively, after the optimization of ZnO NW dispersion is achieved.

TH-2495_146102016